1. Field of the Invention
The invention relates to a code reading pattern and an image pickup apparatus for reading the pattern. The code reading pattern is formed on an object such as a semi-conductor wafer in order to represent information.
2. Description of the Prior Art
FIG. 23 is block diagram of a conventional image pickup apparatus for a code reading pattern as shown, for example, Japanese laid-open patent publication No.2-198128. In the figure, the numeral 12 denotes a white light source, the numeral 13 a floodlight system, the numeral 14a, 14b, 14c an incident light, the numeral 15a a code section pattern, and the numeral 15b a base portion. The numeral 16b denotes the reflected light from the code section pattern 15a and the numeral 16a, 16c is the reflected light from the base portion 15b. The numeral 17 denotes an image lens, and the numeral 18 a light receiver.
The operation of the conventional system is explained below. The code reading pattern which represents the information of the object such as a semi-conductor wafer is formed by the material having a different reflectance from the base portion 15b, or formed by scraping the whole code uniformly. The code is illuminated obliquely by the white light source 12 and the floodlight system 13, and the image of the code is picked up by imaging the reflected light at the light receiver 18 such as a line sensor and a CCD camera through the image lens 17, as shown in FIG. 23. As described above, the code is picked up using the intensity difference between the reflected light 16b and the reflected lights 16a, 16c.
Since the conventional code reading pattern and the conventional image pickup apparatus for reading the code pattern are constructed as described above, the surface state such as the reflectance and the roughness of the base portion 15b change according to the respective production process or respective device. Therefore, there is a state where there is almost no intensity difference between the reflected light from the code section pattern 15a and base portion 15b. In this case, it is very difficult to recognize the code reading pattern because of difficulty for picking up the code at a stable picture quality. In that case, there is provided a method which carries out the marking process of the pattern for each production process in order to identify the pattern itself.
There is another method which provides a complex and large-scale image processing and a change of light angle in order to increase the ability of the pattern recognition at the image pickup apparatus side. But, in both cases there is a problem that the cost for reading the code becomes expensive.